1. Field of the Invention
The invention concerns a catalytic converter which has a metallic monolith that is mounted by a heat-insulating blanket or mat of refractory ceramic fibers. More generally, the invention is concerned with any device that has a canister containing a metallic member and a heat-insulating blanket between the metallic member and the canister.
2. Description of the Related Art
A catalytic converter typically has a metallic or ceramic monolith which is mounted within a metal canister by a heat-insulating blanket. The blanket typically is a mat of refractory ceramic fibers. In use, the monolith is heated to temperatures on the order of 600.degree.-1000.degree. C. while the heat-insulating mat keeps the canister at much lower temperatures. This temperature differential causes a metallic monolith to expand more than the canister, thus narrowing the gap between the monolith and the canister and compressing the heat-insulating mat. Unless the mat is resilient, it can take such a compression set that the metallic monolith becomes loose when the catalytic converter cools. This would not only reduce the heat-insulating value of the mat, but in vehicular use, a loose monolith would produce annoying noises and by being buffeted would be subject to premature failure.
In addition to catalytic converters, there are other devices wherein a canister or other housing contains a metallic member that becomes hot and must be insulated from the housing by a blanket or mat of refractory ceramic fibers, e.g., a diesel particulate trap, an insulated end-cone, or an insulated doublewalled exhaust pipe such as that of coassigned U.S. Pat. No. 5,024,289 (Merry). Any such heat-insulating blanket or mat should be sufficiently resilient that it does not become loose by taking a compression set when the inner metallic member expands in use to narrow the gap between the inner metallic member and its housing.
A mat of refractory ceramic fibers that has sufficient resiliency to prevent a metallic monolith from becoming too loose is disclosed in coassigned U.S. Pat. Nos. 4,929,429 and 5,028,397 (both Merry). The refractory ceramic fibers used in the mat of the Merry patents can be made from an aqueous solution or a colloidal dispersion that is called an "organosol" or a "sol gel". The Merry patents say that substantially shot-free ceramic fibers, formed by sol gel processes, offer the high degree of resiliency needed for mounting metallic monoliths, but that conventional ceramic fibers formed by melt processes contain shot particles and are not suitable. Comparative examples of the Merry patents show that even when melt-formed ceramic fibers have been treated to reduce the shot content to as low as 5%, they still lack the requisite resiliency. Untreated melt-formed ceramic fibers typically have shot contents within the range of 30 to 60%.
Refractory ceramic fibers formed by sol gel processes can be either crystalline or amorphous, depending upon the temperature at which they are fired. Those formed by conventional melt processes are initially amorphous. UK Pat. Spec. No. 1,481,133 (Johnson et. al.) says that a blanket of amorphous ceramic fibers will retain a substantially permanent set under compression, but that good resiliency can be achieved by converting from an amorphous form to a fine-grained crystalline form. This can be achieved by heating above the devitrification temperature of about 950.degree. C., while avoiding higher temperatures (above about 1050.degree. C.) that would result in a coarse-grained structure. We fail to find any indication of use in the Johnson patent specification, but as of its original filing date (1974), mats or blankets of refractory ceramic fibers were. commonly used as furnace liners.
U.S. Pat. No. 4,312,911 (Smith et. al.) concerns fibers of a refractory compound selected from the group consisting of silica, alumina, aluminum silicate, titania, titanium silicate, zirconia, zirconium silicate and mixtures thereof, which fibers can be either amorphous or microcrystalline. The Smith patent specifically concerns improving the heat and shrink resistance of such fibers (and of articles such as mats made from the fibers) by coating them with Cr.sub.2 O.sub.3. The shrink resistance can be further improved by pretreating the fibers at a temperature between about 1150.degree. C. and 1350.degree. C. for from 1 to about 10 minutes.
U.S. Pat. No. 4,055,434 (Chen et. al.) concerns fibers made from a melt of relatively pure alumina and silica plus from 3 to 16% by weight of either burnt dolomite or calcium oxide and magnesium oxide.